1. Technical Field of the Invention
This invention relates to magnetic recording media and, particularly, flexible magnetic recording sheets stretched across and adhesively attached to opposite major surfaces of a disk-shaped support to provide planar recording surfaces.
2. Description of the Prior Art
Conventional, commercially available linear tape recording systems have excellent fidelity characteristics, are durable and are relatively inexpensive. The main shortcoming of linear tape systems is that access to information located on widely separated portions of the tape requires a relatively large amount of time in winding and rewinding the tape.
A disk configuration, on the other hand, will allow random access to different portions of the recorded information since all of the information is available on a single plane. In addition, a plurality of magnetic transducer heads may be used in conjunction with a disk to provide rapid access to random portions of the information recorded on the disk. Thus, while the disk configuration functions the same as the linear tape configuration in that audio or video signals are imparted to a magnetic, iron-oxide coated film and reproduced from this film by means of a transducer head as the iron-oxide coated film is transported relative to the head, the disk configuration has greater utility than the tape configuration because the information recorded on the disk is located in a single plane. This allows the transducer head to rapidly access widely separated portions of the recorded material, an advantage which is not possible with linear tape configurations.
One type of recording disk presently in wide-spread use is the so-called "floppy" disk, such as illustrated in U.S. Pat. No. 3,668,658 issued to Flores et al. This disk type consists of a relatively thick sheet of polymeric material which includes a magnetizable surface coating. Floppy disks are both inexpensive and convenient to use. Drawbacks associated with this type of disk, however, are that the disk experiences relatively large dimensional changes in response to temperature and humidity fluctuations and that a pressure pad opposite the transducer head is required which causes wear, limits the rotational speed and, hence, reduces access times.
The development of stretched surface recording (SSR) disks has progressed over the years from a rather simple configuration described in U.S. Pat. No. 3,373,413, issued to Treseder, in which a film was stretched and clamped between two circular rings, to a dish-shaped support to which a stretched film was attached at the periphery as illustrated by U.S. Pat. No. 3,509,274, issued to Kihara. Subsequently, U.S. Pat. No. 3,537,083, issued to Voth, introduced the concept of bonding the film at the center of the support in addition to the support periphery, and U.S. Pat. No. 3,599,226, issued to Lips, described an SSR disk which included two stretched film surfaces on either side of the support which were attached at the periphery and the center of the support.
Although the patents described above have illustrated the general configuration and many desirable features of SSR disks, many practical problems yet remain as impediments to the mass production and general acceptance of SSR disks as a recording medium. These include the problems of economically and reliably attaching a very thin film to a relatively thick substrate, ensuring that the film remains at a desired tension and planar with respect to the support, thermal expansion differentials between the support and the film, reduction of usable recording area by the area necessary for bonding the film to the support, and the useful life of the bond between the film and the support.
The major problems associated with SSR disks is ensuring that the films remain planar and achieving a reliable bond between the film and the substrate. The film may be maintained in a planar condition by providing the substrate with raised, radially inward and outward ridges which project above the major surfaces of the substrate and provide reference surfaces across which the film may be stretched. It was thought that a reliable bond between the film and the substrate could be achieved by providing annular, sloped film attachment surfaces inward and outward of the inner and outer annular ridges, respectively, which were below the level of the reference surfaces produced by the raised ridges. It was found, however, that a flat sloped surface did not yield an adequate adhesive bond between the film and the substrate because of a peculiar property of annular films bent around an annular support.
If an annular film is bent around an annular fulcrum located near the inner diameter of the film, the film assumes a downwardly concave contour between the fulcrum and the tool used to pull the film around the fulcrum. The film at a fulcrum located near the outer diameter of the annular film assumes a downwardly convex contour between the fulcrum and the tool used to stretch the film around the outer fulcrum. Thus, the contour of the film as stretched around the reference surfaces described above will not conform to a flat bonding surface, even if this bonding surface is sloped downwardly with respect to the plane of the substrate. It has been found that this contour effect of the stretched film is related to the diameter of the annular fulcrum around which the film is stretched and is more severe as the diameter of the annular fulcrum decreases.